Naturally occurring and synthetic smectite clays such as bentonite, montmorillonite, hectorite, fluorohectorite, saponite, beidellite, nontronite, and related analogs, are generally comprised of layers which can be visualized as a sandwich or platelet containing two outer sheets of silica tetrahedra and an inner or central octahedral sheet of a hydrous metal oxide, such as alumina or magnesia octahedra. These platelets are stacked one upon the other to yield a clay particle. Normally, this arrangement yields a repeating structure (d.sub.001) about every ten Angstroms to give a material with relatively low surface areas.
The alkali metal and alkaline earth metal ions in ordinary smectite clays can be replaced by ion exchange with polyoxymetal cations. Polyoxycations in the size range of from about 4 to 10 Angstroms function as molecular props or "pillars", and serve to keep the clay layers separated in the absence of a swelling solvent. Thus, whereas the layers of an ordinary smectite collapse in van der Waals contact when heated to elevated temperatures to give a material with low surface area (approximately 10-30 m.sup.2 /g), smectites pillared by polyoxycations can exhibit d.sub.001 distances in the 14.5 to 20 Angstrom range at temperatures to 500.degree. C., as well as surface areas in the range of from 200 to 400 square meters per gram.
General background respecting pillared clay products and processes for producing same can be found in a number of issued United States patents, including among others, U.S. Pat. Nos. 4,271,043 and 4,248,739 to Vaughan et al, which relates to methods for preparing pillared interlayered clays by reacting a smectite type clay such as a bentonite with high molecular weight cationic metal complexes, such as those of aluminum, zirconium or titanium.
Similarly, U.S. Pat. No. 4,216,188 to Shabria et al discloses preparation of a montmorillonite which is cross-linked with aluminum hydroxide or with chromium hydroxide.
U.S. Pat. No. 4,410,751 to Shin et al in part discloses a catalyst comprised of a smectite such as a montmorillonite, having zirconium oxide intercalated to form pillars between sheets of S.sub.i O.sub.4 tetrahedra of adjacent aluminosilicate layers.
Of particular interest in relation to pillared interlayered clays of the type heretofore discussed, are those in which chromium-based complexes are interlayered--this in view of the fact that oxides of chromium in catalysts are known to demonstrate high catalytic activities.
In the aforementioned Shabria et al, U.S. Pat. No. 4,216,188, chromium hydroxide is thus disclosed as a pillaring agent. The hydroxide treating solution in Shabria is prepared and aged at room temperatures; and the resultant pillared clay samples, even without being subjected to high temperature heatings, have repeating (d.sub.001) spacings of the order of about 18 Angstroms. Specific surface areas are in the range of about 160 to 240 m.sup.2 /g, which is relatively low.
Further to be noted is that G. W. Brindley and S. Yamanaka, Amer. Mineral., 64, 830-835 (1979) have reported a chromium interlayered montmorillonite. This product had d.sub.001 =17.7 A at 25.degree. and 16.5 A at 200.degree. , contained 1.88 Cr/unit cell, and exhibited a surface area of 280 m.sup.2 /g when dehydrated in air at 200.degree. C.
In U.S. Pat. No. 4,452,910 to Hopkins et al expanded layer smectites having a regular pore structure and suitable for catalytic uses, are prepared by a process wherein the clay is treated with a chromium-oligomer solution and subjected to a stabilization heat treatment in an inert gas atmosphere. The chromium-oligomer solutions in Hopkins et al are prepared at room temperatures, and aged at such temperatures. The resultant pillared clay products have repeating spacings d.sub.001 of the order of 15 Angstroms or less, and surface area of from 44 to 305 square meters per gram, where the treated smectite is a bentonite.
Of additional relevance to this invention is our copending application Ser. No. 641,912 filed Aug. 17, 1984, wherein we have disclosed a process for preparing delaminated smectite clay compositions having bimodal pores, which facilitate diffusion of large hydrocarbon molecules. As disclosed therein, the process includes reacting a mixture of a trioctahedral smectite having an average particle size of up to about 500 Angstroms, and in general a lath-shaped morphology, with a polymeric cationic hydroxy metal complex in an aqueous medium, and recovering the reacted clay solids, preferably by freezedrying. Smectite clay compositions prepared in accordance with the said procedure have a completely or substantially completely "delaminated" structure. In particular, the predominantly edge-to-edge and edge-to-basal association of the clay layers which result yields a combination of macro- meso- and microporosity in the clay, which has high thermal stability. The product exhibits an extremely diffuse or substantially non-existent X-ray diffraction pattern.